"New Signing" card — maintainer export for a contributor's first merged PR

[tobiager]

Context

Idea: when a contributor lands their first significant PR on a repo, the repo's maintainer can generate a Real-Madrid-presentation-style card to post: "🚨 OFFICIAL DEAL: @username joins the @owner/repo squad! Transfer Fee: €75k. Welcome to the pitch!" — a shareable, celebratory image, distinct from the existing squad exports which show the whole XI, not one new signing.

This is genuinely new surface area, not a variant of an existing route — scope it as such rather than trying to force-fit it into the existing SquadExportPanel.

Files involved (starting points, not a fixed list — this needs its own design)

• New render function, likely lib/svg-squad/render.ts (or a new renderNewSigningCard) and/or a new OG route
  under app/api/og/squad/[owner]/[repo]/signing/ (or similar) — model the params/caching approach on
  app/api/og/squad/[owner]/[repo]/route.tsx.
• "Transfer Fee" needs a real number — the obvious source is the player's own marketValue (lib/squad/valuation.ts), not an arbitrary constant; decide how (full value? a fraction, echoing the existing "transfer fee" flavor text elsewhere in the app — see README.md's football↔GitHub mapping table) and document the choice.
• New UI entry point for triggering this — where a maintainer generates it (e.g. from the squad page, gated to "the person is in this squad"). No auth/maintainer-identity system exists in this project yet — the first version can be a plain, unauthenticated generator (like every other export) rather than gated to "verified maintainers"; note that tradeoff explicitly in the PR.

Acceptance criteria

• Given an owner/repo and a contributor login already in that squad, produces a shareable image in the
  existing visual language (fonts/colors/theme system) announcing the "signing."
• A real, explained number for "Transfer Fee" — not a placeholder.
• Has a Copy Markdown / Download / Share flow consistent with SquadExportPanel's existing pattern (reuse
  those components/patterns rather than inventing new ones).
• docs/exports.md gets a section for this new format.

Notes

This is the most open-ended issue in the batch — comment on the issue with your proposed approach (route shape,
where the fee number comes from, where the trigger UI lives) before investing in a full implementation, so scope doesn't drift.